1. Field of the Invention
The present invention relates to a lithographic apparatus and method of securely holding an object in the lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging the pattern using a UV radiation beam onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Conventionally, a clamping device is used to securely hold an object, such as, a reticle or a substrate, to a supporting base structure, referred to as a “stage.” The “stage” is alternatively referred to as a “table,” a “frame,” or a “force frame.” The clamp can be referred to as a “chuck.” A stage may be coupled to a chuck by a variety of means, including kinematic supports at three points. A chuck may also be constructed integral to the stage. During movement of the stage, or during an exposure operation, the object is typically securingly coupled to the chuck using a normal force (i.e., a force acting perpendicular to the chuck) generated by electrostatic attraction or a partial vacuum between the object and the chuck. The normal force and a normal stiffness of the chuck serve to secure the object in a normal direction during the movement or exposure. In a tangential direction, i.e., in a plane of the chuck, the object is prevented from moving relative to the chuck during movement or exposure through friction between the chuck and the object.
When acceleration is imparted to the stage during exposure or pre-exposure alignment, a stress is transferred from the stage to the chuck, and this stress may cause the chuck as well as the object to deform. Typically the transfer of acceleration-induced stress from the stage to the chuck (as well as from the chuck to the object) is not uniform. This gives rise to a potential for slip between the chuck and the object, especially in cases where the chuck deformation is large. Chuck deformation may also be caused by temperature differences between the stage, the chuck, and/or the object, resulting in slippage of the object relative to the chuck.
A conventional approach to limit the transfer of stress between the chuck and the stage is to use precision-machined kinematic or semi-kinematic mounts to isolate the chuck from the stage. However, kinematic mounts at a number of discrete locations may not uniformly distribute the transfer of stress. An alternative approach to distribute the transfer of stress more uniformly is to use a chuck comprising a plurality of burls that make local contacts with the supported object.
FIG. 2A depicts a conventional chuck 200 comprising a plurality of burls 225. A stage 230 holds an object 210, e.g., by electrostatic force applied through a planar electrode 220 in a normal direction. Object 210 has a top surface 212 and a bottom surface 214, which is opposite top surface 212. Burls 225 support object 210. Each of the burls 225 acts like a spring, shown symbolically as spring 205, providing a predetermined amount of shear compliance.
FIG. 2B is a bottom view showing bottom surface 214 of object 210. Top ends of burls 225 form local contacts 227 at bottom surface 214 of object 210.
Dimension and arrangement of burls 225 may be tailored to some extent to provide shear compliance. However, materials used in burls 225 have many additional requirements, including but not limited to, hardness, machineability, coefficient of thermal expansion, etc. Thus, it may be difficult to tailor burls 225 for a desired shear compliance in all directions of interest. For example, while it is desirable to have a high shear compliance at an interface of object 210 and burls 225, it is also desirable to have a low normal compliance at the interface. Burls 225 that directly contact object 210 typically have high compliance in normal direction as well, making it harder to optimize the overall shear compliance of the system. Moreover, burls 225 with the desired shear compliance are typically long and slender, and their shape makes electrostatic clamping of the object quite challenging. Additionally, planarity of object 210 may be compromised due to non-uniform stress distribution in burls 225.